This invention relates to an earth station for carrying out communication in a satellite communication system through a single satellite or a plurality of satellites.
In addition to a satellite or satellites, a satellite communication system comprises a base station and a fixed station. The earth station may be used as a selected one of the base station and the fixed station. Alternatively, a satellite communication system comprises a base station and a movable station. The earth station may be used as one of the base station and the movable station. The movable station is carried by an airplane or an automobile and has a variable location. As a further alternative, the satellite communication system comprises a base station, a fixed station, and a movable station. The earth station may be used as one of the base station, the fixed station, and the movable station.
When used as the movable station which has a small antenna of wide directivity, the earth station is herein called a small earth station. The small antenna has a wide directivity in order to cope with variation of the variable location of the earth station. The satellite communication system generally comprises a plurality of small earth stations. In addition to the satellite communication system, another satellite communication system may use the satellite or satellites and comprise another plurality of small earth stations. In this event, undesirable interference takes place between these satellite communication systems.
In order to avoid such interference, proposal is made about using a spread spectrum technique in U.S. Pat. No. 4,455,651 issued to Paul Barran et al and assigned to Equatorial Communications Company.
The spread spectrum technique is useful for a movable station in locating the variable location at which the station is present. This field of application of the spread spectrum technique is disclosed in U.S. Pat. No. 4,359,733 issued to K. O'Neill.
However, a wide frequency band is occupied when carrying out communication by the use of the spread spectrum technique. This makes it difficult to transmit other data signal through the frequency band and results in a reduction of efficiency of transmission.
In order to improve the efficiency of transmission, the frequency band is divided into a lower frequency band and a higher frequency band. The lower frequency band is employed for frequency division multiplexed signals. The higher band is employed for spread spectrum signals. Inasmuch as only the higher frequency band is used, the spread spectrum signals are unavoidably received with a reduced gain.